Beyond-Diagonal RIS For Enhanced Secrecy and Sensing Gains in Secure ISAC Networks: An Optimization Framework

Integrated sensing and communication (ISAC) has been receiving a notable interest as an energy- and spectrum-efficient enabler for simultaneous communication and sensing.

Notably, reconfigurable intelligent surfaces (RIS) is among the key technologies enabling robust communication and sensing, particularly in environments without a line-of-sight (LoS).

Recently, a new type of RIS, called beyond-diagonal RIS (BD-RIS), has drawn attention, offering additional degrees of freedom in controlling the propagation medium.

In this paper, a novel secure BD-RIS-aided ISAC scheme is proposed and evaluated.

The scheme is applicable to a multi-user multi-target ISAC network, where a dual-functional radar-communication (DFRC) base station (BS) simultaneously serves multiple downlink users and senses various targets that aim to eavesdrop on the legitimate signal transmitted to the users.

The presence of a BD-RIS enables circumventing the absence of the LoS link and ensures secure transmission and sensing.

To this end, an optimization problem is formulated aiming at maximizing a weighted sum of per-target reflected powers, subject to secrecy and transmit power constraints.

Thus, by virtue of an Augmented Lagrangian- and Riemannian conjugate gradient-based approach, in addition to semidefinite programming, an alternating optimization (AO)-based algorithm is developed, which provides a local optimum for the BD-RIS scattering matrix, transmit signal beamforming matrices, and artificial noise covariance matrix.

Numerical results highlight (i) the notable sensing gains of the BD-RIS-aided design with respect to its diagonal RIS (D-RIS)-based baseline and (ii) the improved secrecy-sensing trade-off, whereby the BD-RIS can ensure an increasing system secrecy without a significant loss in the per-target reflected power.